Did we go over this earlier in this thread or in the other one?
Trusting a photo or video might work the same way you trust a website today. Upon opening an HTTPS socket connection, a website presents a valid(not expired, not outdated SSL version) certificate signed at its root by a trusted certificate authority.
Devices that are intended to produce trustworthy photos and videos will need to purchase certificates from a certificate authority the same way a website does today. Signing anything is essentially a mathematical hashing algorithm, thus a certain photo signed with a certain certificate should be computationally unique. Thus, you can use this to prove a photo is authentic.
How do you deal with a photo's certificate expiring shortly after it's taken(website certificates are usually only valid for a year)? No idea.
How do you deal with hashing algorithms being cracked and reverse engineered? No idea.
It all boils down to an arms race in the end. While having to prove a video or photo is authentic is perhaps new, having to prove something digital is authentic is not.